Enkephalins, the endogenous ligands for opiate-receptors, have been discovered in various parts of brain in many vertebrates. Information regarding the possible role of these opioid-peptides is only beginning to accumulate. It is believed that these ligands may act as neurotransmitters or modulators in the central nervous system. Most of the pharmacological and physiological studies have centered around the role of these ligands in analgesia. Since most of their actions are similar to those of narcotic analgesics, it is expected that they may have adverse effects on respiration and circulation. Information regarding the effects of enkephalins on the respiratory and cardiovascular systems is fragmentary. In the present study, two parenterally active enkephalin-analogues will be used to investigate the actions of enkephalins on these two physiological systems. If these two analogues turn out to have less undesirable side effects than morphine, it will be an important step forward in the search for safer and effective analgesics. If the results are contrary, it will be helpful in designing future enkephalin analogues for safer analgesia. In addition, the results of this investigation may extend our knowledge regarding the physiological role of enkephalins.